It is often desirable to provide a signal simultaneously in multiple beams of a multibeam antenna system. For example, a cellular communication system may provide communications between a base transceiver station (BTS), having an antenna system associated therewith, and a plurality of mobile units operating within a predefined area, or "cell," defined by the antenna system's radiation pattern. Often such cells, although providing communications in a full 360.degree. about the BTS, are broken down into three 120.degree. sectors in order to provide more capacity and less interference over that of an omni cell 360.degree. system. Additionally, such a sectorized cell achieves extended range as compared to an omni cell 360.degree. system due to the greater signal gain at the sector antennas resulting from their more focused coverage.
Further advantage may be realized by providing multiple narrow beams at the BTS rather than the three 120.degree. sectors. For example, twelve 30.degree. narrow antenna beams may be utilized to provide the same 360.degree. communication coverage within the cell as the 360.degree. omni cell configuration and its 120.degree. sectorized cell replacement. Such a multiple narrow beam arrangement is desirable because, as with the 120.degree. sector system described above, the multiple beams provide a greater signal gain resulting from their greater focused coverage. A further advantage of the multiple narrow beams is the flexibility offered in synthesizing any desired sector size. Combining adjacent narrow beams provides a wider composite beam, with a beam width roughly equal to the sum of the individual beams widths. Accordingly, synthesized sectors may be formed of any size from a full 360.degree., by simulcasting a signal on each of the narrow beams, to as small as the narrow beams themselves, by providing the signal only within one narrow beam.
However, it should be appreciated that there is a potential for phase nulling associated with simulcasting of identical signals within multiple beams. For example, it may be necessary to synthesize a 120.degree. sector pattern comprised of four 30.degree. beams. Conceptually the 120.degree. sector may be synthesized by simply simulcasting the desired signal (CDMA waveform or AMPS signaling channel) over the four contiguous 30.degree. narrow beams making up the desired 120.degree. radiation pattern. However, phase differences between the signals radiated by the four constituent beams can cause signal cancellation. This cancellation leads to undesirable shaping, i.e. "holes," in the composite radiation pattern. For example if the antennas creating the narrow beams are separated by several wavelengths, deep nulls occur in the resulting antenna pattern, giving it a "rippled" appearance azimuthally. This pattern is not desirable for a BTS as it implies that there are "holes" in the coverage corresponding to the nulls in the pattern.
This phase nulling problem created by simulcasting is an artifact of the multibeam approach and is potentially a problem regardless of the type of signal transmitted, i.e., phase nulling is not unique to digital systems such as with respect to dynamic beam mapping in code division multiple access (CDMA) systems, but also exists with respect to signalling in analogue systems such as the advanced mobile phone system (AMPS) and narrowband advanced mobile phone system (NAMPS). Therefore, it is desirable to identify a solution that addresses both CDMA and AMPS/NAMPS. However, due to dynamic beam mapping desirable in CDMA, there may be some unique aspects associated with CDMA.
A need therefore exists in the art for systems and methods by which signals may be provided to a multibeam antenna system for simulcasting over multiple ones of the antenna beams without producing undesirable nulls.
A further need exists in the art for the systems and methods to avoid nulls in the simulcasting over multiple antenna beams of digital signals as well as analogue signals.
A still further need exists in the art for the systems and methods avoiding nulls to be adapted so as not to adversely affect forward link performance.